Ion microanalyzer

ABSTRACT

An ion microanalyzer employing a mass spectrometer of a double focussing type in which a specimen is enclosed by a shield body, and variable voltage is applied to the specimen and the shield body to make the acceleration voltage for secondary ions variable.

Tamura et al.

[451 Oct. 29, 1974 lON MICROANALYZER Inventors: Hiiumi Tamura, l-lachioji;

Kazumitsu Nakamura, Katsuta; Toshio Kondo, Sagamihara, all of Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Mar. 20, 1973 Appl. No.: 343,019

Foreign Application Priority Data Mar. 21, 1972 Japan 47-27386 US. Cl 250/306, 250/309, 250/397 1m. Cl. HOlj 37/26 Field of Search 250/309, 306, 396, 397

PRIMARY-ION IRRADlATION DEVICE [56] References Cited UNITED STATES PATENTS I 3,646,344 2/l972 Plows 250/310 3,686,499 8/1972 Omura 250/309 Primary Examiner-James W. Lawrence Assistant ExaminerC. E. Church Attorney, Agent, or FirmCraig & Antonelli [5 7] ABSTRACT An ion microanalyzer employing a mass spectrometer of a double focussing type in which a specimen is enclosed by a shield body, and variable voltage is applied to the specimen and the shield body to make the acceleration voltage for secondary ions variable.

15 Claims, 5 Drawing Figures RECORDER Pmmmm m4 v FIG. I PRIOR ART PRIMARY-ION IRRADIATION DEVICE Pmmmmm 3.845304 sum 20F s F 2 PRIMARY-ION R IRRADIATION DEVICE ,2

, a A k L 7:: g g M f9 RECORDER J RECORDER ION MICROANALYZER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improvement in an ion microanalyzer.

2. Description of the Prior Art The ion microanalyzer of the prior art has a problem in that it is significantly hard to discriminate between the secondary ions emitted from atoms of the specimen and those from vapor phase or absorption layer. This results in reduction of accuracy or difficulty in identifying elements. Therefore, this problem should, by all means, be solved when considering microanalysis of element and analysis of thin film or surface of material.

SUMMARY OF THE INVENTION It therefore is an object of this invention to provide an improved ion microanalyzer capable of discriminating between the secondary ions emitted from the specimen and those from vapor phase.

Another object of this invention is to provide an ion microanalyzer of a single focusing type ease rarefiecriminating between the secondary ions emitted from the specimen and those from the vapor phase.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of an ion microanalyzer of the prior art.

FIG. 2 is aschematic diagram of one embodiment of the ion microanalyzer in accordance with the present invention. I

FIG. 3 is a schematic diagram of another embodiment of this invention.

FIGS. 4a and 4b show the relationship between secondary ion energy and mass spectrum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For easy understanding of this invention, description will first be made about an ion microanalyzer of the prior art.

In FIG. 1, there is shown an ion microanalyzer of the prior art. The prior art analyzer is composed of a primary ion irradiation device 1 including an ion source, means for focussing an ion beam 2, and means for deflecting the ion beam (not shown); a specimen 3; an electrode 4 for attraction of secondary ions; at mass spectrometer 5, 6, 7 and 8; a secondary ion detector 9; and a recorder 10. Conventional massspectromete'rs are usually divided into a single focussing type which is provided with only the sector magnetic field device 7, and a double focusing type one which is provided with the sector electric field device and with the sector magnetic field device 7. The former type one is used in such a case that a relatively poor resolution is permitted because it cannot perform an energy selection of the secondary ions emitted from the specimen. The latter type one is used in case a high resolution is required. In an analysis which is conductedby using the mass spectrometer of a double focusing type, a voltage of several KV or less is applied to the specimen while a voltage of about 10 percent of that applied to the specimen is applied to both the upper and lower electrodes of the sector electric field device.

In the microanalyzer of FIG. 1, a secondary ion acceleration voltage Va and the voltage iVd to be applied to the sector electric device are supplied by individually independent voltage sources 12, 13 and 14. In some apparatuses the voltages are variable while keeping the ratio therebetween constant. In the former microanalyzer, the secondary ion acceleration voltage i.e., the specimen potential Va, is maintained constant, and the energy selection of the secondary ions is made by changing the voltage forming the sector electric field. In the latter microanalyzer, the secondary ion acceleration voltage Va and the sector potential iVd are concurrently changed so that energy selection of the secondary ion cannot be done. The former microanalyzer is capable of energy selection, but has a difficulty in maintaining the absolute value of the secondary ion energy constant. The reason for this is that change of the voltage applied on both the electrodes of the sector electric field device destroys the condition for the double focusing. Consequently,-it is necessary to adjust the sector magnetic field so that the double focusing condition is satisfied each time the voltage applied to the sector electrode is changed. Accordingly, in this case, estimation on the energy of the secondary ions is made by calculation, and the accuracy of the determination of the energy is reduced depending on the primary ion' beam irradiating position, the path of the beam, etc.

From the above description, it may be seen thatthe ion microanalyzer of the prior art has inherently a problem when a high accuracy is required in identifying elements and in quantitative analysis. This invention is directed to solving the problem and to achieving an improvement in the ion microanalyzer.

A detailed explanation of the present invention will now be described hereinbelow.

The improvement attained by this invention maybe summarized as follows: g

l. The specimen" is shielded by a shielding body which is formed of a conductive mesh or perforated conductor electrode, and an appropriate voltage is applied between the specimen and the electrode;

2. The specimen voltage, i.e., the secondary ion acceleration voltage Va can be made variable independent of the voltage for the sector electric field.

Basically, the ion microanalyzer in accordance with the present invention will operatein two modes: I. The one is such that only the secondary ions having energy larger than apredetermined energy are guided to the mass spectrometer by applying the electric field between the specimen and the shield body electrode for An embodiment of the latter mode will first be described with reference to the schematic diagram in FIG. 2. Si and GaAs single crystals were employed as the specimen '3. The specimen chamber is evacuated to l X 10' Torr, and then air was introduced therein until the degree of vacuum in the chamber reaches 1 X 10 Torr. The specimen was irradiated by ion beams wherein ion current was lU uA, the acceleration voltage was KV, and argon ions Ar were used as primary ions. THe specimen voltage, i.e., the secondary ion acceleration voltage Va was set at 3 KV while the output of the mass spectrometer was adjusted equal to the mass peak of the sealed-in gas, i.e. 0+ or N+. The sector electrical field and the sector magnetic field then are adjusted so as to minimize the 0 or N The measurement was made of the mass spectrum of the secondary ions emitted from gas phase in such a manner that after such adjustment, only the exciting current for the sector magnetic field is changed with the other portion being fixed. Next, the acceleration voltage of the secondary ion is reduced so that the ions of high energy alone may be introduced into the mass spectrometer. The mass spectrum of the bulk is measured by changing the exciting current for the sector magnetic field.

FIG. 3 shows another embodiment of this invention in which this invention is applied to the mass spectrometer of a single focusing type. The mass-spectrometer of this kind in the prior art has inherently no ability to select the energy of the secondary ions, but has a high utility value in that the secondary ions are utilized in high efficiency and the device is simple in the construction. Thus, a gross effect will be expected if the single focusing type could be capable of selecting the energy of secondary ions. The energy selection of the secondary ions is achieved by changing the potential of the electrode relative to the specimen by tVg which is supplied through a switch 17 from a voltage source 16.

Consider now the case in which positive ions are derived as the secondary ions. In this case, the potential ofthe electrode 15 is made higher by Vg than the specimen potential. Accordingly, the secondary ions having an initial energy of less than Vg (eV) can not pass through the electrode 15 while the ions of Vg (eV) or larger are permitted to pass therethrough into the mass spectrometer.

On the other hand, in case negative ions are to be de rived, the potential of the electrode 15 is made lower, by Vg than the specimen voltage. As a result, only the secondary ions of over Vg are introduced into the mass spectrometer.

The experiment of this invention will next be explained. The results from the experiments in accordance with the above described methods of energy selection are approximately identical. Thus, a description will be made only about the mass spectrometer of a double focusing type.

in this experiment, Si single crystal is employed as the specimen. FIGS. 4a and 4b show the results of the experiment.

The mass spectrum as shown in FIG. 4a is observed in the detection of the secondary ions of relativelylow energy (10 KV). It can be seen from the drawing that the mass spectrum of C*, CH, N*, 0 0*, etc. emitted from the gas phase are found, and that of the atom of the specimen (Si) is not found. The observed spectrum is not changed even when the specimen is substituted by others. Thus, it is. apparent that this spectrum shows the secondary ions from the gass phase.

On the other hand, in the experiment in which the secondary ions of high energy are detected, it is observed as shown in FIG. 4b that gas ions of the gas phase disappear or are reduced, and the secondary ions emitted from the specimen (Si) are detected. It is to be noted that the mass spectrum which is the combination of those shown FIGS. 4a and 4b, is observed in the conventional device.

From the above description, it is apparent that the present invention provides an improved ion microanalyzer which is capable of remarkable discrimination between the secondary ions emitted from the specimen and the gas phase ions of low energy.

While the present invention has been shown and described in a few terms only, it will be apparent that various modifications may be made without departing from the spirit and scope thereof.

We claim:

1. An ion microanalyzer comprising: means for generating an ion beam, means for directing said beam to a desired portion of a specimen, a mass spectrometer for analyzing secondary ions emitted from said speci- 20 men, a variable electric source for applying a voltage to said specimen, and a shield body which encloses said specimen and to which said voltage is applied.

2. An ion microanalyzer comprising:

means for generating an ion beam;

means for directing said beam to a desired portion of a specimen;

a mass spectrometer for analyzing secondary ions emitted from said specimen;

an electric source for applying a voltage to said specimen;

a shield body which encloses said specimen; and

means for applying a desired potential difference between said shield body and said specimen.

' 3. An ion microanalyzer as claimed in claim 2, in which said mass spectrometer is of a single focusing type provided with a sector magnetic field device.

4. An ion microanalyzer comprising:

first means for generating an ion beam;

40 a specimen;

second means for directing said beam to a desired portion of said specimen;

a mass spectrometer for analyzing secondary ions emitted from said specimen; an electric source for applying a voltage to said specimen;

a shield electrode which encloses said specimen; and

third means for electrically connecting said specimen with said shield electrode.

5. An ion microanalyzer according to claim 4, wherein said mass spectrometer is of a single focusing type provided with a sector magnetic field device, and

said third means includes an electric source applying a voltage between said specimen and said shield electrode.

6. An ion microanalyzer according to claim 5, wherein said third means includes means for reversing the polarity of said voltage. 0 7. An ion microanalyzer according to claim 4, wherein said mass spectrometer is of a double focusing type provided with a sector electric field device, a sector magnetic field device, and a variable electric source for applying a voltage to said sector electric field de- 9. An ion microanalyzer according to claim 4, wherein said shield electrode is a perforated conductive electrode.

10. An ion microanalyzer according to claim 7, wherein said shield electrode is a conductive mesh.

11. An ion microanalyzer according to claim 7, wherein said shield electrode is a perforated conductive electrode.

12. In an ion microanalyzer having first means for generating an ion beam and directing said ion beam onto a desired portion of a specimen;

second means, connected to said specimen, for applying a prescribed voltage thereto; and

a massspectrometer for analyzing secondary ions emitted from said specimen as a result of said ion beam being directed thereon;

6 the improvement comprisingi a shield electrode body enclosing said specimen; and 

1. An ion microanalyzer comprising: means for generating an ion beam, means for directing said beam to a desired portion of a specimen, a mass spectrometer for analyzing secondary ions emitted from said specimen, a variable electric source for applying a voltage to said specimen, and a shield body which encloses said specimen and to which said voltage is applied.
 2. An ion microanalyzer comprising: means for generating an ion beam; means for directing said beam to a desired portion of a specimen; a mass spectrometer for analyzing secondary ions emitted from said specimen; an electric source for applying a voltage to said specimen; a shield body which encloses said specimen; and means for applying a desired potential difference between said shield body and said specimen.
 3. An ion microanalyzer as claimed in claim 2, in which said mass spectrometer is of a single focusing type provided with a sector magnetic field device.
 4. An ion microanalyzer comprising: first means for generating an ion beam; a specimen; second means for directing said beam to a desired portion of said specimen; a mass spectrometer for analyzing secondary ions emitted from said specimen; an electric source for applying a voltage to said specimen; a shield electrode which encloses said specimen; and third means for electrically connecting said specimen with said shield electrode.
 5. An ion microanalyzer according to claim 4, wherein said mass spectrometer is of a single focusing type provided with a sector magnetic field device, and said third means includes an electric source applying a voltage between said specimen and said shield electrode.
 6. An ion microanalyzer according to claim 5, wherein said third means includes means for reversing the polarity of said voltage.
 7. An ion microanalyzer according to claim 4, wherein said mass spectrometer is of a double focusing type provided with a sector electric field device, a sector magnetic field device, and a variable electric source for applying a voltage to said sector electric field device.
 8. An ion microanalyzer according to claim 4, wherein said shield electrode is a conductive mesh.
 9. An ion microanalyzer according to claim 4, wherein said shield electrode is a perforated conductive electrode.
 10. An ion microanalyzer according to claim 7, wherein said shield electrode is a conductive mesh.
 11. An ion microanalyzer according to claim 7, wherein said shield electrode is a perforated conductive electrode.
 12. In an ion microanalyzer having first means for generating an ion beam and directing said ion beam onto a desired portion of a specimen; second means, connected to said specimen, for applying a prescribed voltage thereto; and a mass spectrometer for analyzing secondary ions emitted from said specimen as a result of said ion beam being directed thereon; the improvement comprising: a shield electrode body enclosing said specimen; and third means electrically connecting said shield electrode body to said specimen.
 13. The improvement according to claim 12, wherein said third means comprises means for applying a selected voltage between said specimen and said shield electrode body.
 14. The improvement according to claim 13, wherein said selected voltage is a variable voltage.
 15. The improvement according to claim 12, wherein said third means comprises means for directly electrically connecting said specimen to said shield electrode body. 